The present invention relates to methods for producing a reflective optical element for an operating wavelength in the soft X-ray and extreme ultraviolet wavelength range, in particular for use in an EUV lithography apparatus, which has a multilayer system composed of at least two alternating materials having different real parts of the refractive index at the operating wavelength on a substrate, which exerts a stress on the substrate, and in which there is arranged between the multilayer system and the substrate a layer of material, wherein the thickness thereof is dimensioned in such a way that the stress of the multilayer system is compensated for, and also to methods for producing a reflective optical element for an operating wavelength in the soft X-ray and extreme ultraviolet wavelength range, in particular for use in an EUV lithography apparatus, which has a first multilayer system composed of at least two alternating materials having different real parts of the refractive index at the operating wavelength on a substrate, which exerts a layer stress on the substrate, and which has a second multilayer system composed of at least two alternating materials having different real parts of the refractive index at the operating wavelength on a substrate, which exerts an opposite layer stress on the substrate and is arranged between the first multilayer system and the substrate.
In addition, the invention relates to reflective optical elements produced by these methods. Furthermore, the invention relates to a projection system and an illumination system and also to an EUV lithography apparatus comprising at least one reflective optical element of this type.
In EUV lithography apparatuses, reflective optical elements for the extreme ultraviolet (EUV) or soft X-ray wavelength range (e.g. wavelengths of between approximately 5 nm and 20 nm) such as, for instance photomasks or multilayer mirrors are used for the lithography of semiconductor components. Since EUV lithography apparatuses generally have a plurality of reflective optical elements, the latter have to have a highest possible reflectivity in order to ensure a sufficiently high total reflectivity. Since a plurality of reflective optical elements are usually arranged one behind another in an EUV lithography apparatus, even relatively minor impairments of the reflectivity for each individual reflective optical element already affect the total reflectivity within the EUV lithography apparatus to a relatively large extent.
Reflective optical elements for the EUV and soft wavelength range generally have multilayer systems. These are alternately applied layers of a material having a higher real part of the refractive index at the operating wavelength (also called spacers) and of a material having a lower real part of the refractive index at the operating wavelength (also called absorbers), wherein an absorber-spacer pair forms a stack or a period. This in a certain way simulates a crystal whose network planes correspond to the absorber layers at which Bragg reflection takes place. The thicknesses of the individual layers and also of the repeating stacks can be constant over the entire multilayer system, or else vary, depending on which reflection profile is intended to be achieved.
Within multilayer systems, as early as during the coating process a stress can build up which acts on the underlying substrate and deform it to such a great extent that the optical imaging at the corresponding reflective optical element is critically disturbed. The type of stress depends, inter alia, on the materials used as spacers and absorbers and the thickness ratios present within a stack or a period. A measure of this thickness ratio is defined as , the ratio of absorber layer thickness to total thickness of a period. The basic profile of the stress as a function of  is illustrated schematically for a multilayer system on the basis of molybdenum as absorber material and silicon as spacer material in FIG. 5. For operating wavelengths in the range of approximately 12 nm to 14 nm, the highest reflectivities are obtained if reflective optical elements whose molybdenum-silicon multilayer systems have a  in the region of approximately 0.4 are used. A compressive stress is to be expected there. A tensile stress is to be expected at higher values of . The concrete relationship between the stress in the multilayer system and the  value, that is to say in particular the gradient and the precise position of the zero crossing, in this case depends on the choice of the coating process and the respective coating parameters.
The relationship between stress and  can be utilized to produce stress-reduced reflective optical elements. For this purpose, there is arranged between the substrate and the multilayer system optimized for a high reflectivity at the respective operating wavelength a further multilayer system, which is optimized in particular through the choice of an appropriate  for the purpose of as far as possible compensating for the stress of the highly reflective multilayer system or minimizing the total stress within the reflective optical element. However, it should be taken into consideration that in the case of the coating processes suitable for the production of reflective optical elements for the soft X-ray and extreme ultraviolet wavelength range, namely magnetron sputtering, ion-beam-assisted sputtering and electron beam evaporation, with conventional coating parameters, starting from a certain thickness, a crystallization of the respective layer occurs particularly in the case of the absorber layer. In the case of molybdenum, for example, said crystallization already occurs starting from a layer thickness of approximately 2 nm. The crystallite sizes increase as the layer thickness increases, which leads to an increase in the microroughness and hence the surface roughness. At the high  values necessary for the stress reduction, it is already possible to ascertain a roughening which in total, that is to say over the entire stress-reducing multilayer system, brings about an appreciable increase in the roughness at the surface of the stress-reducing multilayer system. Since this roughness also continues in the overlying highly reflective multilayer system, both the reflectivity and the optical imaging of the reflective optical element deteriorate. This deterioration is usually avoided by choosing the  of the stress-reducing multilayer system to be small enough to avoid a roughening, and in return providing a larger number of periods of the stress-reducing multilayer system in order to sufficiently compensate for the stress. However, this entails the disadvantage of a coating process having an increased time duration and hence also a greater risk of failure as a result of incorrect coating.